Cellulose esters have been used since their discovery for manufacturing products with a strong aesthetic appeal. Among them, a material particularly widespread in the eyewear and fashion jewelery industry is cellulose acetate.
The strengths of said polymers are thermoplasticity, transparency, colourability, malleability, as well as the physical-mechanical features which make them suitable in the eyewear industry, optical industry, fashion jewelery industry and more recently in the industry of technological instruments.
Typically, this material is supplied to product manufacturers in the shape of semi-processed sheets, which already have their characteristic colour (with varied patterns which are perceived in transparency in the sheet depth) and which are subsequently simply cut to obtain the final products (for example a glasses frame).
The processes known for obtaining the sheet are, among others, the wet block process (for example the material manufactured under the trade names XELOX®, XELOX-S®, RHODOID® or M49® available from the same applicant), extrusion and co-extrusion (for example the material manufactured under the commercial names XELOX-T®, OPTIROID® available from the same applicant), the dry block process (for example the material manufactured under the commercial names CEBLOX® or TECBLOCK® available from the same applicant), injection and co-injection. Through typical processing it is possible to impart to such sheets a strong aesthetic appeal, by melting powders (enriched with plasticisers and solvents), granules, semi-finished products of various shapes and thicknesses among which also the sheets.
Historically, the first aesthetic effects which were imparted to the sheets were the ones borrowed from nature. As a matter of fact, through the processing of cellulose esters it was possible to obtain synthetic copies of animal-origin raw materials such as horn, tortoise-shell, bone, but also wood.
A block processing, which allows to obtain these aesthetic effects, is disclosed in EP267409, owned by the same applicant. The technique disclosed in this patent has been widely used over the years with great effectiveness and satisfaction. It provides to evenly and randomly mix, although with varying relative amounts, two different types of basic granular elements (such as granules, powders, dice, fragments, . . . ) and then subjecting them to pressure and heat, so that they become inextricably bonded, creating an effect of generalised diffusion of the dark colouring towards the lighter material part.
However, this technique allows to obtain substantially even sheets, that is, with an evenly distributed minute design (with geometric evenness or randomly) across the sheet. That does hence not allow to obtain, within the same sheet, a blurred colour hue or other more marked colour changes between predetermined different areas of the sheet.
In many circumstances the market requires instead sheets which allow to obtain blurred effects between one colour and another, in geometrically predefined areas. For example, in the eyewear industry, it is desired to be able to offer eyeglasses in which the frontpiece has a darker colour pattern in the upper part and a lighter colour pattern in the lower part (or vice versa) or a darker colour pattern on the two sides and lighter in the central area (or vice versa), wherein there is a certain continuity between the different patterns.
In this context, by “pattern” any layout of one or more colours is understood, which defines, in its relevance area, an even aesthetic appearance, for example a single-colour, or a so-called “Havana” or even a diced/chequered effect, a horn effect, other geometric/fancy patterns and so on.
So far, in order to be able to obtain differently-coloured sheets, one can proceed in two different ways: through special extrusion techniques (see, for example, documents U.S. Pat. No. 2,985,556 and U.S. Pat. No. 3,513,060 where extrusion processes are detailed, while also U.S. Pat. No. 3,288,666 discloses an example of how the extruded sheet with a coloured wedge find an application) or with gluing/thermal or chemical welds of different semi-finished products.
As can be guessed, both these techniques are not fully satisfactory, both because they are complex, and because they do not allow to obtain a perfect blending effect between one pattern and the other. As a matter of fact, between one pattern and the other more or less dividing lines nevertheless remain (especially in the case of gluing) which may not be aesthetically appealing. The joining lines sometimes originate also actual local deformations (due to differentiated shrinkage of the materials or to the discontinuity caused by the joining means).
In the semi-finished products obtained by these known-art solutions, some effects are precluded (for example, with the extrusion and injection methods some spots cannot be produced; by contrast, with the standard block methods, obtaining wedges or inserts would imply additional processing, with the uncertainty of the final effect), while others are extremely labour-intensive or costly. Moreover, in the sheet from which eyeglasses (or other products) are obtained or in the roughly-shaped blocks for eyeglasses (or other finished product), the gluing/joining area gives origin to some flaws as well as making the finished product more fragile. In some cases the gluing or the joining may be visible, creating rifling effects or effects of aesthetic flaws, said flaws being due to the dissimilar dimensional behaviour of the mutually joined various components. In order to overcome these flaws, it has been proposed to resort to the welding of different sheets along inclined surfaces, according to planes inclined by few degrees (below 30°) with respect to the observation surface of the sheets. That allows to obtain a blurred colour effect, exploiting the semi-clearness of the material which, in the direction of sheet depth, causes a sensation of blurring. However, these techniques are not fully satisfactory either.
It must furthermore be pointed out that the joining processes of different materials is typically carried out on small-sized semi-finished products, such as plates of the order of magnitude of a glasses frame. That prevents from obtaining scale economies in processing speed, which may instead be achieved if there was a wide-surface sheet in which to automatically cut a plurality of identical imprints.
Finally, it must be considered that the different processes and compositions which originate the two different-pattern materials significantly characterise the semi-finished products, and can hence determine also a behaviour differentiated in time (shrinkages, colour changes, surface appearance, . . . ), which makes progressively manifest in an undesirable way the different nature of the various components.
Similar problems occur in sheet obtained by lamination of several layers of different colours.
It would hence be desirable to be able to offer a process for obtaining thermoplastic sheets of consistent material which is as homogeneous as possible, wherein at least two different patterns or aesthetic effects are defined, perfectly distinguishable from each other but joined not by clear dividing lines but rather by blurred areas.